Re: logic related question ...

In order to understand what you need to do, you need to realize that each step is a transformation of the previous step by some instance of the provided laws. Which law do you use? That is for you to figure out by simple pattern matching. As already pointed out to you, the first step is a direct application (instance) of Law 3. Do you not see why? If you replace the generic "a" and "b" in Law 3 by the specifics given in the problem "p" and "q", you get exactly the transformation we see from our assumption to step 1.

Now think about step 2. What has changed between step 1 and step 2? Which of the provided laws represent that change?

Re: logic related question ...

You are right, the commutative law of disjunction allows us to transpose positions of each disjunct. The point of the law is to generalize a schema of something that is true for every similar instance. While in your case you are dealing with "¬p v q" it would also work for complex statements like "(p & r) v q" (i.e., it is equivalent to "q v (p & r)"). The point of these exercises is to become familiar with what the laws entail. The name of the law helps explain that. Commutitive laws allow you to commute (transpose, switch positions, etc.) terms around the connective. The law of (double) negation allows you to add a double negative or take it away. The law of implication gives you an equivalent form that is useful for conjunctive or disjunctive normal forms. The law of equivalence lets you break up a biconditional into conjunctions (and from the conjunct of implication with law 3 you can have a conjunction of disjunctions).

Now do you see how the laws justify each transformation at each step? More importantly, do you see why each step is performed due to the provided law?

Re: logic related question ...

Why do you think that? Does it fit the pattern of behavior I described each of the laws to have as you quoted above? Note, you're not using every law in that proof. Some of them are used more than once. Often a pattern of reasoning is like that: you put the statement in other terms that make it easier to put into another form that can then be put into other terms that are desired. That is what is going on here. Remember, your end goal of the proof is to show that ¬q → ¬p. Just as before, what has changed between steps 2 and 3? What is changed between 3 and 4? That change is indicative of some law, otherwise you would not be able to make that sort of transformation. If you can understand that pattern of change, you can answer my original question as to why the law is used at that step. If you cannot answer the 'why' question, you have not grasped the exercise. The point of it is to obtain that pattern recognition skill and relate it to your knowledge of the logical operations involved.

Re: logic related question ...

So if I understand correctly they want you to start with the first step (¬p ∨ q) where you see almost immediately that this belons to Law 3 ... only the characters are different.
And the end result should be ¬q ⇒ ¬p
ANd we get there with using the different laws in the correct order right?

So, in step 2, we go from ¬p ∨ q to q ∨¬p. Why do you think that this is described by law 2: ¬(¬a) ⇔ a rather than by law 1: (a ∨ b) ⇔ (b ∨ a)? Does the starting formula, i.e., ¬p ∨ q, look more like ¬(¬a) or like a ∨ b?